Yup, be honest with them. Let them know that you understand that they aren't sure, but unfortunately, from your point of view, you can't just "wait and see". You need to know, because you have a family lined up that could be filling that space. I would give them a week to decide, and let that other family know that the situation is dependent upon the choice. Then you can move forward.
I don't think that it's a "putting your foot down" situation, where you need to get mean or anything. They probably just haven't thought about it from that point of view. And it's not unfair to ask for a yes or no.
PS- good work on the Canadianish, eh??
And congrats on the interest in your daycare space!